This invention relates to composite materials formed by thermoplastic resins and reinforcing fibers. More particularly, this invention relates to such composite materials and the process to form such materials, where the thermoplastic and fiber components impart great strength and commercial utility to the composite compared with materials made previously to this invention.
The forming of composite materials using polymers and fibers has been known heretofore, and the processes, methods, apparatus, and products relating to such composites are disclosed in the patents of Montsinger (U.S. Pat. No. 5,176,775) and Cogswell (U.S. Pat. No. 4,541,884). The difficulties of wetting fiber with high viscosity thermoplastic resins are well understood. Montsinger in ""775 discloses a method of counter current flow between polymer and continuous fiber roving to increase shear and lower viscosity of thermoplastic resin. Cogswell relies on low viscosity polymers.
The present invention produces high strength, composite materials with thermoplastic polymer and reinforcing fiber. The thermoplastic polymer may have a high molecular weight resulting in a high viscosity. The fiber may be continuous rovings of glass, carbon, metal, and/or organic filaments. The commercial products of this invention may include long fiber reinforced thermoplastic compounds which are reformable and injection molding pellets. Injection molding pellets have a fiber length equal to the pellet length, generally about 0.5 inches, and also have a generally circular cross section with an aspect ratio (length/diameter) greater than 1. Another product of this invention is continuous fiber reinforced thermoplastic profiles which have a constant shape such as a round rod and may be advantageously used in structural applications in place of metal because of the relative low weight and high strength.
The polymer and fiber composite materials are produced in a fiber melt impregnation process in which continuous filament fiber is contacted with molten polymer and extruded or pultruded through a die orifice. In such a process, the exit orifice size of the die usually determines the fiber loading of the composite. For example, increasing the exit area allows more polymer to pull through the die with a given fiber amount thereby lowering the fiber concentration. According to the present invention, relative rotation is imparted between the exit die and the advancing polymer impregnated fiber. The relative rotation can be achieved, for example, by rotating the exit die about the axis of the advancing fibers. This produces an unexpected increase in the fiber concentration and the benefit of improved strength of the composite compared to products made without rotation.
The exit die, preferably having a conical entrance and cylindrical exit, is rotated about the axially directed rovings of continuous fiber. Polymer melt is conveyed by pressure flow and/or fiber drag flow into the orifice chamber. The rotation of the conical chamber wall induces a conihelical flow path for the polymer due to viscous drag from wetting of the wall by the polymer in addition to the axial drag by the fiber. The polymer flow path becomes helical in the cylindrical region of the rotating orifice chamber. The difference in velocities and directions between the polymer and fiber moving in an axial direction and the polymer moving in a vortiginous, conihelical direction give rise to a dispersive and impregnative shear to wet out fiber with polymer. In addition the polymer chains are coiled around the reinforcing, axially directed fiber and bound polymer to create a normal stress effect of polymer backflow. The velocity distributions are represented in FIG. 1 as described in more detail below.
The present invention thus provides a method of producing a fiber reinforced thermoplastic material which comprises directing continuous filaments along a predetermined advancing path of travel into and through an impregnation chamber. Molten thermoplastic polymer material is injected from an extruder into the impregnation chamber and into intimate contact with the advancing filaments for wetting and impregnating the filaments with the thermoplastic material. The filaments are then pulled through the exit die from the exit end of the impregnation chamber while the exit die is rotated about the axis of the advancing filaments.
The present invention also provides an apparatus for producing a fiber reinforced thermoplastic material, the apparatus including an impregnation chamber having an entrance end and an exit end. The exit end includes an exit die having a die opening. Means are provided for directing continuous filaments along a predetermined advancing path of travel into and through the impregnation chamber so that the filaments enter through the entrance end and exit through the exit die. An extruder provides a supply of molten, thermoplastic polymer. Means is provided for directing the molten thermoplastic material from the extruder into the impregnation passageway and into intimate contact with the filaments. This promotes wetting and impregnation of the filaments with the molten thermoplastic polymer. Means are provided for rotating the exit die about the axis of the filaments.
A fiber reinforced thermoplastic composite produced with this process and apparatus has a unique structure with a core region and a skin region. The size of each region is related to the pulling speed and orifice rotation speed. The skin is composed only of polymer. Rotation of the orifice reduces the volume of polymer in the skin by contributing to the core region and normal stress polymer backflow. The core is composed of unidirectionally aligned fibers which are fully impregnated with the matrix thermoplastic polymer. The core polymer appears to be more crystalline than the skin polymer probably because of slower, internal cooling. The polymer chains in the core are aligned with the fiber by the fiber drag. The skin is oriented by the helical flow alignment of the polymer chains. Shear strength measurements of composite strands showed greater strength with increasing rotation speed at a given line speed and fiber concentration. Inner laminar shear strength of the composite was also improved.